A) 117
B) 121
C) 123
D) 129
Correct Answer: C
Solution :
3,9, 15,......21st term. \[\Rightarrow 3\times 1,\text{ }3\times 3,\text{ }3\times 5,\text{ }....\text{ }3\text{ }\times \text{ }21st\] odd number. \[\Rightarrow\] 21st term \[=3\times \left( 2\times 21-1 \right)\] \[-3\times 41=\text{ }123.\] (Note nth odd number is \[\left( 2n-1 \right)).\]You need to login to perform this action.
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